1. Field of the Invention
The present invention relates to an electric power steering apparatus, and more particularly it relates to improvement in motor driving while a steering wheel is returning to its initial position.
2. Description of Related Art
In an electric power steering apparatus widely used these days, a motor is driven by a current value corresponding to detected steering torque, and the thus obtained driving force of the motor is transferred to a steering mechanism so as to assist the steering power.
FIG. 1 is a circuit diagram showing the configuration of a motor control circuit in a conventional electric power steering apparatus. FETs Q.sub.1, Q.sub.2, Q.sub.3 and Q.sub.4 form a bridge circuit between a power supply E and a ground terminal. A motor M and a resistance R for detecting a motor driving current are connected in series so as to bridge the contact between the FETs Q.sub.1 and Q.sub.3 connected in series and the contact between the FETs Q.sub.2 and Q.sub.4 also connected in series. The voltages at both ends of the resistance R for detecting a motor driving current are respectively inputted to the non-inverted input terminal and the inverted input terminal of a differential amplifier 4, which outputs a differential output generated in accordance with the inputted voltages as a signal I.sub.d being equivalent to the motor driving current. Further, the FETs Q.sub.1, Q.sub.2, Q.sub.3 and Q.sub.4 contain free wheeling diodes D.sub.1, D.sub.2, D.sub.3 and D.sub.4 connected thereto in parallel, respectively.
A value of steering torque supplied to a steering wheel. (not shown) and detected by a torque sensor 5 is inputted to a signal processor 3, which outputs, in accordance with the detected torque, a signal 11 being equivalent to a target motor current (i.e., a target value of a motor current) to the non-inverted input terminal of a differential amplifier 1. To the inverted input terminal of the differential amplifier 1 is inputted the signal I.sub.d being equivalent to the aforementioned motor driving current. The differential amplifier 1 outputs to an FET driving circuit 2 a difference between these inputted values as a signal V.sub.1 being equivalent to a target motor voltage required for supplying the target motor current to the motor M. In the FET driving circuit 2, a motor driving voltage V.sub.N is generated from the signal V.sub.1 being equivalent to the target motor voltage. By using a duty ratio in accordance with the magnitude of the absolute value of the motor driving voltage V.sub.N, and also in accordance with the polarity of the motor driving voltage V.sub.N, the FET driving circuit 2 turns on the FET Q.sub.3 and simultaneously performs a pulse width modulation control (hereinafter referred to as PWM control) on the FET Q.sub.2, or turns on the FET Q.sub.4 and simultaneously performs the PWM control on the FET Q.sub.1, thereby driving the motor M in the clockwise or counterclockwise direction.
While a steering wheel (not shown) is returning to its neutral position, however, the driving direction of the motor M (i.e., the direction in which the motor is to be rotated) in accordance with the steering torque detected by the torque sensor 5 is different from the actual rotation direction of the motor M. Accordingly, a generated current caused by the rotation of the motor M flows through the circuit, resulting in causing a control hunting. As a result, not only an abnormally loud noise is made in the motor M but also a braking force in the reverse direction of the rotation direction of the steering wheel is generated, thereby preventing the steering wheel from returning to its neutral position smoothly.
Such a phenomenon will now be described in more detail. In returning the steering wheel to its neutral position, if the motor M is rotated in, for example, the counterclockwise direction while driving in the clockwise direction, the FET Q.sub.2 is PWM controlled and the FET Q.sub.3 is turned on. FIG. 2 is a circuit diagram illustrating the operation of the bridge circuit in a general motor control circuit. As is shown in FIG. 2, when the PWM controlled FET Q.sub.2 is on, a PWM controlled current flows through the FETs Q.sub.2 and Q.sub.3 as shown with a solid line arrow 6. When the PWM controlled FET Q.sub.2 is turned off, a generated current caused by the rotation of the motor M and a regenerated current caused by the inductance of the motor M both in the direction shown with a broken line arrow 7 flow through the FET Q.sub.3 and the diode D.sub.4. When the generated current is increased, the differential output of the differential amplifier 4, i.e., the signal I.sub.d being equivalent to the detected value of the motor driving current, is increased to exceed the signal I.sub.1 being equivalent to the target motor current (i.e., the non-inverted input has a larger absolute value than the inverted input). Then, the polarity of the differential output of the differential amplifier 1, i.e., the signal V.sub.1 indicating the target motor voltage, becomes reverse of that of the signal I.sub.1 being equivalent to the target motor current. When the polarity of the signal V.sub.1 indicating the target motor voltage becomes reverse, the FET driving circuit 2 makes the motor driving voltage V.sub.N zero, thereby turning off all the FETs including the FETs Q.sub.2 and Q.sub.3. Accordingly, no current flows through the resistance R, and the differential output of the differential amplifier 4, i.e., the signal I.sub.d being equivalent to the detected value of the motor driving current, becomes zero. As a result, the polarity of the differential output of the differential amplifier 1, i.e., the signal V.sub.1 being equivalent to the target motor voltage, regains its original polarity.
When the signal V.sub.1 being equivalent to the target motor voltage regains the original polarity, the FET driving circuit 2 generates the motor driving voltage V.sub.N corresponding to the signal V.sub.1 being equivalent to the target motor voltage, and performs the PWM control on the FET Q.sub.2 and turns on the FET Q.sub.3 again. When the FET Q.sub.2 is PWM controlled and the FET Q.sub.3 is turned on, a PWM controlled current flows through the FETs Q.sub.2 and Q.sub.3, and a generated current caused by the rotation of the motor M flows through the FET Q.sub.3 and the diode D.sub.4 again. The generated current is increased, thereby increasing the differential output of the differential amplifier 4, i.e., the signal I.sub.d being equivalent to the detected value of the motor driving current to exceed the signal I.sub.1 being equivalent to the target motor current. That is to say, the inverted input has a larger absolute value than the non-inverted input. Then, the polarity of the differential output of the differential amplifier 1, i.e., the signal V.sub.1 being equivalent to the target motor voltage, becomes reverse again. In this manner, such a process is repeated, resulting in disadvantageously causing the control hunting and making an abnormal noise in the motor M.
Next, a motor lock check circuit will be described. FIG. 3 is a block diagram showing the configuration in the major part of the motor lock check circuit in the conventional electric power steering apparatus. The motor lock check is an operation to check whether or not the motor in the electric power steering apparatus normally works (i.e., whether or not the motor is locked) when an ignition switch is turned on. FETs Q.sub.1, Q.sub.2, Q.sub.3 and Q.sub.4 form a bridge circuit, and a motor M and a resistance R for detecting a motor driving current are connected in series so as to bridge the contact between the FETs Q.sub.1 and Q.sub.3 connected in series and the contact between the FETs Q.sub.2 and Q.sub.4 connected in series. The FETs Q.sub.1, Q.sub.2, Q.sub.3 and Q.sub.4 are connected with free wheeling diodes D.sub.1, D.sub.2, D.sub.3 and D.sub.4 in parallel, respectively. The bridge circuit, a fail safe relay 24 and a power supply B are connected in series.
After an ignition switch (not shown) is turned on, the fail safe relay 24 is turned on by a controller (not shown). In an off condition of an electromagnetic clutch (not shown) for transferring the driving force of the motor M to a steering assisting mechanism, a signal processor 23 turns on, for example, the FETs Q.sub.1 and Q.sub.4 via an FET driving circuit 20, and drives the motor M for a predetermined period of time by a driving current flowing in the direction shown with a solid line arrow 25. Then, the signal processor 23 turns off all the FETs including the FETs Q.sub.1 and Q.sub.4, and allows the motor to rotate by the inertial force. A motor terminal voltage detecting circuit 22 detects a generated voltage caused by this rotation of the motor M. Then, the signal processor 23 outputs a signal for braking the motor M to the FET driving circuit 20 before the electromagnetic clutch is turned on. The FET driving circuit 20 turns on the FET Q.sub.2 in response to the signal for braking the motor M, and the motor M is stopped by a regenerated current that is generated by the generated voltage caused by the inertial rotation of the motor M and flows through the free wheeling diode D.sub.1 and the FET Q.sub.2 in the direction shown with a broken line arrow 26.
FIG. 4 is a timing chart showing the variation in the motor current during the motor lock check. In this timing chart, T.sub.1 indicates a driving time of the motor M, when a driving current flows as shown with the solid line arrow 25; T.sub.2 indicates a time required for detecting a generated voltage caused by the inertial rotation of the motor M, when no current flows through the bridge circuit because all the FETs are off; and T.sub.3 indicates a braking time of the motor M, when a regenerated current flows in the direction shown with the broken line arrow 26.
In this manner, in a conventional motor lock check circuit, a generated voltage caused by the rotation of the motor M is detected by the motor terminal voltage detecting circuit 22, which detection requires the time T.sub.2.